Geologica Romana 37 (2003-2004), 1-66 LATE TOARCIAN - LATE AALENIAN AMMONITES ASSEMBLAGE FROM MT. MAGAGGIARO (WESTERN SICILY, ITALY) Giovanni Pallini*†, Serge Elmi** & Francesca Gasparri*** * Università degli Studi di Chieti “Gabriele D’Annunzio”, Via dei Vestini, 27 - Chieti ** Université “Claude Bernard” Lyon 1, UFR des Sciences de la Terre, 27-43 Bd. du 11 Novembre 1918, 69622 Villeurbanne Cedex -France, e-mail: [email protected] *** Via Alessandria, 154 - 00198 Roma, e-mail: [email protected] ABSTRACT - This study describes a condensed and reworked ammonites assemblage found in a quarry loca- ted south of Mt. Magaggiaro (Trapani). The specimens were collected from a pocket filled by multiple polymetal- lic crusts and stromatolites and sandwiched between shallow-water platform sediments (carbonates of the Sciacca Formation and Inici Formation) and overlying pelagic deposits (limestones and marly limestones - the Ammonitico Rosso Formation). All the specimens (145 representing 29 genera, 3 subgenera, 39 species and 15 subspecies) have normal species size; adult specimens of large dimensions are also present. The ammonites are very well preserved, someones with iridescent shells. The species range from the Meneghinii Zone to the Concavum Zone (Late Toarcian-Late Aalenian). The assemblage is very similar to that collected by Vacek (1886) and De Gregorio (1886 b), differing however for the presence of many Phylloceratids, rare at Cape San Vigilio, whereas other forms (Dumortieria spp. and Polyplectus spp.), abundant in Sicily, lack at Cape San Vigilio. Oppelids are not present in Sicily. A seaway probably existed, connecting the two areas (today > 1000 km apart) and allowing for an unconstrained pattern of areal distribution of ammonite species. Several species are also in common with the assemblages from the margins of the Middle Atlas basin in Morocco, in similar condensed, iron oxide-rich strata. The recovery of representatives of Pleydellia and Vacekia is especially remarkable, as they belong to species that have been recorded rarely from Italy. There is a strong predominance of Hildoceratids and Graphoceratids that constitute together almost 57% of the collected specimens. The presence of the Meneghinii (Dumortieria spp.), Aalensis (Pleydellia spp.) and Opalinum (Leioceras and Costileioceras) Zones is established. The Murchisonae Zone is documented by the association of different species of Vacekia, Abbasites, Abbasitoides and Erycites. The Bradfordensis Zone is recognized through the association of Pseudographoceras, Brasilia, Eudmetoceras (= “Pseudaptetoceras” christianae). Ptychophylloceras chonomphalum and Eudmetoceras eudme- tum indicate the Concavum Zone. Our condensed layer is directly overlain by a bed bearing an assemblage with Parkinsonia parkinsoni and Cadomites deslongchampsi which suggests a hiatus encompassing the whole Early Bajocian through the Parkinsoni Zone (Late Bajocian). KEY WORDS: Ammonites, Jurassic, Aalenian, Sicily, Italy. GEOLOGICAL SETTING metres of platform limestones and dolomites bearing Late Triassic megalodontids; The present paper describes a Toarcian-Aalenian - Inici Formation (called also “Calcari Bianchi”), consti- ammonites assemblage found in a thin condensed level tuted by 200-300 m of Lower-Middle Liassic peritidal (which caps the peritidal carbonates of the Inici and open platform limestones; Formation, and is in turn overlain by nodular pelagic - limestones and marly limestones, generally referred to limestones), which exhibits several interesting sedimen- as the Ammonitico Rosso Formation, with condensed tological and paleotectonic features (Di Stefano et al., ammonite-rich levelzs, ranging in age from the Early 2002). The study area belongs to the Mt. Magaggiaro Jurassic (“Calcari a Crinoidi” of the Domerian/Toarcian- structure, in SW Sicily, near the town of Menfi (Fig. 1). Aalenian) through the Early Tithonian (“Calcari a Mt. Magaggiaro is an anticline having a south-western Bositra”, “Calcari nodulari”, “Calcari a Pygope”). vergence, and is part of the Mt. Magaggiaro-Pizzo In the northern part of the quarry, the platform sediments Telegrafo Unit. It belongs to one of the Mesozoic paleo- (Inici Formation) and their pelagic cover are separated geographic domains of Sicily, the so-called Saccense by a condensed level, also bearing multiple polymetallic domain. crusts, which has yielded the ammonites described in The lithostratigraphy of the area has been recently this article (Figs. 2-3). Stromatolites, Belemnites, described by Di Stefano et al. (2002), and will be briefly Gastropods, Nautiloids (Nautilus and Pseudaganides) summarized below (stratigraphic bottom to top): (Pl. 1, Figs. 1, 2, 2a, 6) and Bivalves are also present. - Sciacca Formation, constituted by several thousands of 2 Geologica Romana 37 (2003-2004), 1-66 PALLINI et al. Fig. 1 - Outcrop location (red star) (from Wendt 1963 modified) IF: Favignana Island; MA: Mt. Arancio; MB: Mt. Bonifato; ME: Mt. Erice; MG: Montagna Grande; MK: Mt. Kumeta; MI: Mt. Inici; MP: Mt. Pispida; RB: Rocca Busambra; RP: Rocca che parla. PREVIOUS STUDIES ic correlations. The association collected and studied by Gemmellaro has recently been revised by several The earliest studies about the ammonites assemblages authors (Cecca et al., 2002) and Vacek’s material was of Sicily are those by Gemmellaro (1886), Tagliarini revised by Callomon et al. (1994). Some Aalenian (1901), Renz (1925), Christ (1960) and Wendt (1963). species are also found in several Apennines sections, but These Authors described rich assemblages sampled in those particular ammonites were not collected in situ very thin condensed levels, which apparently do not (Cresta et al., 1995 cum bibl.). Similar assemblages are allow to separate individual chronozones. Cresta (1997), also known from the Middle Atlas of Morocco (Benshili, in his revision of some of the Hammatoceratid species 1989; El Arabi et al., 2001; El Hammichi, 2002). studied by Gemmellaro, stresses how the Sicilian associ- ation is as important as the one described at Cape San Vigilio (Southern Alps) by Vacek (1886). Its importance BIOSTRATIGRAPHIC INTERPRETATION OF is not stratigraphic but rather paleontologic, because THE REWORKED AMMONITES ASSEMBLAGE there are species of Hammatoceratids which are missing at Cape San Vigilio, whereas other ones, that make up A geo-sedimentary interpretation of the Jurassic suc- 50% of the assemblage in Sicily (i.e. Erycites fallifax), cession at Cava D’Anna, along with a preliminary pale- are missing in Gemmellaro’s collection as well as in our ontological report, can be found in Di Stefano et al. outcrop. Cresta (1997) states that Gemmellaro’s species, (2002). characterizing the Mediterranean paleoprovince in the In the quarry, the top of the Inici Formation is an ero- Late Toarcian-Aalenian, can be used for paleogeograph- sional surface truncating gently rotated beds. This bed tilting is interpretated as a product of synsedimentary Fig. 2 - The Cava D’Anna outcrop. LATE TOARCIAN – LATE AALENIAN AMMONITES ... Geologica Romana 37 (2003-2004), 1-66 3 carenite matrix; the nature of the pebbles is diverse (grey, yellow or brown micrites, some containing rock fragments and ammonites). Irregular erosional surfaces are locally seen in the conglomerate, which produced ammonites ranging in age from the Late Toarcian to the Late Aalenian. The bioclastic calcarenites which consti- tutes the matrix of this deposits is seemingly the product of infilling of large vertical or sub vertical sheet cracks, and is thus a later addition to the ammonites bed. Pelagic material is infilled in the bioclastic calcarenite through some successive vertical dykes. Finally we suppose that there are at least two generation of calcite crusts: before the ammonites level and after its deposition; - a centimetric crust, formed by thin laminae, mainly composed of Fe and Ca; Mn concentration is variable; - a stromatolithic crust. Stromatolithic laminae caps Fig. 3 - A crust fragment with ammonites. ammonites whatever their size. extension, and the quarry exhibits some spectacular Fossil preservation ranges from poor to excellent. This examples of listric faults and associated rollover within can be a product of the following causes: the platform carbonates of the Inici Fm. The topmost - ammonites may have been buried slowly and pre- beds of this formation are clearly seen to seal these served only as internal casts (usually light-red coloured); growth structures. Extension predated erosion and - ammonites may have been rapidly embedded and drowning of the platform. This latter produced a switch their mineralised test is still preserved; they bear the in the sedimentary regime to condensed pelagic deposi- common features of condensed fossils (also with tion, which took place in a structural-high setting geopetal structures); they can be coloured in dark red to Sciacca Plateau, Santantonio (2002). Our ammonites brown or black; level is, with the exception of thin laterally discontinu- - ammonites may have been “wrapped” and protected ous pockets of crinoid/brachiopod/belemnite-rich lime- by brown-yellowish stromatolithic domes, preserving a stone, the lowest pelagic deposit resting on the drowning thin, film-like pseudo-test. unconformity. It is a laterally discontinuous condensed Tests are often colonized by annelids, ostracods and horizon, dark brown in colour, characterized by an crinoids, especially in the umbilical area. As these impressive accumulation of cephalopods shells and by ammonites come